Friction welding comprises using relative motion between a tool and a workpiece, or between first and second workpieces, to generate heat to plastically displace and fuse the material of the workpiece without melting the material of the workpiece. As the material of the workpiece is not melted, friction welding is known as a “solid state” welding method.
Several types of friction welding or known, including linear friction welding, rotational friction welding and friction stir welding (FSW).
In linear friction welding, the relative motion is a lateral oscillation between the first and second workpieces. In rotational friction welding, the relative motion is rotational between the first and second workpieces. In both lateral and rotational friction welding, the first and second workpieces are joined together at the end of the welding process.
In friction stir welding (FSW) a tool is rotated relative to the workpiece about an axis of the tool. The tool comprises a probe and a shoulder either side of the probe. During FSW, the tool is rotated about the tool's axis at high speed and is “plunged” into the material such that the probe extends beneath the surface of the material. The tool is then moved laterally along a join between two adjacent workpieces. Material in the workpieces adjacent a contact surface between the probe and the workpieces is heated, plasticised and mixed by contact with the tool in a “weld-zone”. The weld zone advances along the workpieces to form a weld as the tool is moved along the join. In FSW, two adjacent workpieces are joined together, but the tool is generally removed at the end of the welding process. In FSW, it is desirable that the tool does not become part of the joined workpieces, since the tools are relatively expensive.
FSW has been used successfully for welding workpieces made of aluminium for example. However, FSW of some materials, such as titanium alloys has been found to be difficult, since tools suitable for other metals may fail when used for FSW of titanium, particularly during the plunge. The tool may be worn away such that the outer diameter of the probe is reduced, or may shatter.
One solution is to use a tool comprising a tungsten based alloy such as that described in UK patent application 2402905. However, such tools are relatively expensive, and are consumed (i.e. worn away, and possibly incorporated into the weld) by the welding process. It has been found that the length of material that can be welded before failure using such tools to weld titanium is of the order of 3 to 5 meters. If the tool fails before the weld is complete, the workpiece will generally have to be discarded, as the weld will generally have to be continuous in order to obtain the required strength. Since the tool is incorporated into the weld, the weld may not be as strong as the parent materials of the workpiece. There is therefore a requirement for a welding tool which can be made more cheaply, and which can achieve longer weld lengths, while resulting in a weld having close to the same strength as the parent material.
The present invention seeks to overcome some or all of the above problems.